The present invention is concerned with radar display apparatus particularly for an azimuth scanning radar. It is already an established technique in radar apparatus to digitise the received video return signals. Such digitised video can then be processed using digital techniques which can have many advantages. It is also well known to employ scan converters for converting the normal rotating PPI radar display to a T.V. type display. Such T.V. displays can be very much brighter than the rotating PPI displays which have to use a long persistence phosphor to provide a full radar display picture. If the radar video is digitised, digital techniques can be used for this scan conversion and one arrangement for digital scan conversion is described in British Patent Specification No. 1498413.
There have also been various published discussions of the desirability in marine radar of de-correlating sea clutter returns in successive azimuth scans of the radar apparatus, see for example the article "Further Observations on the Detection of Small Targets in Sea Clutter" by Croney Woroncow and Gladman in "The Radio and Electronic Engineer", volume 45, No. 3, March 1975, pages 105 to 115. Indeed, the advantages of integrating slow-moving or stationary target returns over successive azimuth scans are well established. However, hitherto, azimuth scan to scan integration has been achieved using photographic techniques, i.e. multiple exposure on successive aximuth scans of a PPI display. The advantages of azimuth scan to azimuth scan correllation in discriminating small targets in sea clutter, are also discussed in the article "The Impact of New Display Technology on the Detection of Small Targets at Sea by Radar", by Williams, in "Electronic Circuits and Systems", November 1979, Volume 3, No. 6, pages 241 to 246. This article suggests, in the first column of page 244, the use of digital techniques for correllating returns from successive azimuth or aerial scans. However, no detail is given in this article of how these objectives are to be achieved.